Transition-metal cluster compounds have been under scrunity because of their potential catalytic applications. Such clusters are of interest from three perspectives. The first is that they should prove of value as precursors for the preparation of bimetallic and multimetallic heterogeneous catalysts. Such catalysts can be prepared by allowing clusters to absorb on to catalyst supports as SiO.sub.2 and Al.sub.2 O.sub.3, followed by pyrolysis to remove the ligands. The second perspective is that mixed-metal clusters may find application in homogeneous catalysts because of the different reactivities of the metals present in the clusters which may show reactivity patterns different from those of homometallic clusters. Thirdly, the low symmetry of mixed-metal clusters makes them useful for probing various aspects of the reactivity and molecular dynamics of clusters.
Mixed-metal clusters are described by W. I. Gladfelter and G. I. Geoffroy in an article entitled "Mixed-Metal Clusters" appearing in Advances in Organometallic Chemistry, Vol. 18, page 207, G. A. Stone and R. West Editors, Academic Press, New York 1980. There are four general methods for preparing mixed-metal clusters. These are by pyrolysis, addition to coordinately unsaturated compounds, redox condensations and reaction of carbonylmetals with metal halides. Pyrolysis reactions generally involve heating two or mroe stable compounds of different metals to give fragments that then combine to yield the mixed-metal clusters. The addition of coordinatively unsaturated compounds is closely related to the pyrolysis technique and are presumably formed during pyrolysis by dissociation of ligands or cleavage of metal-metal bonds. The redox condensation is the reaction of a carbonylmetalate with a neutral metal carbonyl and has been widely used as a pyrolysis reaction for synthesizing mized-metal clusters. In the reaction of carbonylmetalates with metal halides the carbonylmetalates will displace a halide complex to yield a metal-metal bonded species.